na-adjective, noun
stealthy; secret; quiet and unobtrusive
Describes actions or manner that are done quietly so as not to be noticed; often used with verbs like 動く, 近づく, or 去る.
忍びやかに部屋を出た。
I left the room stealthily.
忍びやかな足音が廊下から聞こえた。
I heard quiet, stealthy footsteps from the hallway.
こっそり is a more common, casual adverb for doing something secretly or on the sly, while 忍びやか is more literary and emphasizes quiet, careful movement.
密か also means secret or stealthy, but is more often used for hidden feelings or plans rather than physical movement.
Derived from the verb 忍ぶ (しのぶ, 'to conceal oneself, to endure') with the adjectival suffix やか, forming a na-adjective meaning 'in a stealthy manner'.